This invention relates to security documents, such as banknotes or the like, and is particularly concerned with providing a security document formed from a plastic substrate with an enhanced security device.
In Australian Patent Specification No. AU-A-87665/82 there is disclosed a security document and a method of producing a security document, in which opacifying coatings of ink are applied to both sides of a sheet-like substrate formed from a clear plastics film. The security document may be produced with some areas to which no opacifying coating is applied on both sides of the clear plastics substrate. These clear, transparent areas are known as xe2x80x9cwindowsxe2x80x9d and are particularly suitable for incorporating security devices, for example diffraction gratings, optically variable devices and embossed images, which can be inspected in the transparent areas or windows from both sides of the security document. However, a security device, such as a diffraction grating, in a window generally has the same appearance when viewed from both sides of the security document.
It is therefore desirable to provide a security document which incorporates a security device that presents a different appearance from opposite sides of the document.
It is also desirable to provide a security document formed from a clear plastics substrate with an enhanced form of security device.
It is further desirable to provide a relatively simple method of providing a security document with an enhanced form of security device.
According to one aspect of the invention, there is provided a security document comprising:
a sheet-like substrate of clear plastics material having first and second surfaces on opposite sides of the substrate;
said substrate having a region including a security device;
at least one layer of opacifying ink applied on said first surface of the substrate and at least partially obscuring said security device from one side of the substrate; and
at least one layer of opacifying ink applied on said second surface on the opposite side of the substrate in such a manner as to leave an area of the second surface which is not covered by the opacifying ink substantially in the region including the security device.
According to another aspect of the invention, there is provided a method of producing a security document comprising the steps of:
providing a sheet-like substrate of clear plastics material having first and second surfaces on opposite sides of the substrate;
said substrate being provided with a security device in at least one region of the substrate;
applying at least one opacifying layer of ink on said first surface of the substrate so as to obscure said security device from one side of the substrate; and
applying at least one opacifying layer of ink to part of the second surface on the opposite side of the substrate in such a manner as to leave an area of the second surface of the substrate which is not covered by ink substantially in the region of the security device.
In the security document and method of the invention, the area of the second surface of the substrate which is not covered by the opacifying ink forms a xe2x80x9chalf-windowxe2x80x9d through which the security device may be inspected from one side of the substrate, the security device being at least partly obscured by the opacifying ink when viewed from the opposite side of the substrate. Thus the security device has a different appearance when viewed from opposite sides of the document.
For instance, the security device may be clearly visible in the half-window from the side of the second surface of the substrate, but not visible or only partially visible, e.g. only in transmission from the side of the first surface of the substrate. Preferably, the security device is more visible in transmitted light than in reflected light from the side of the first surface of the substrate.
The security device may be formed within the transparent plastics substrate. Alternatively, the security device may be applied to at least one of the first and second surfaces of the substrate.
In the method of the invention, the steps of applying the security device and of applying the layers of opacifying ink to the first and second surfaces of the substrate may be performed in different orders. For instance, the step of applying at least one layer of opacifying ink to part of the second surface of the substrate to form the half-window may be performed first before the steps of applying the security device to the substrate in the region of the half-window and of applying at least one opacifying layer to the first surface of the substrate to obscure the security device.
Preferably, however, the step of applying the security device to the substrate is performed prior to the steps of applying the layers of opacifying ink to the first and second surfaces of the substrate which may then be performed either simultaneously or one after another.
Preferably, part of the security device extends transversely outside the area of the xe2x80x9chalf-windowxe2x80x9d which is not covered by the opacifying ink on the second surface of the substrate. Preferably, the part of the security device which extends outside the area of the half-window is more visible in transmitted light than in reflected light. When a security device is provided in a banknote or security document in this manner, different parts of the security device may present different effects to a viewer from each side of the banknote or security document. For instance, if the layer or layers of opacifying ink only partially obscure the security device, the part of the security device outside the half-window area may be visible, at least in transmission, but not as apparent as the part of the security device within the area of the half-window. It is therefore possible for different contrasting effects to be observed between relative visibilities of the different parts of the substrate from both sides of the security document.
A wide variety of security devices may be employed in the present invention. Examples of the type of security devices which may be applied to the substrate in the present invention include: fine line or filigree patterns; micro-text; security stripes or threads; front and rear registration devices, including Moire patterns; embossings; diffracting gratings; optically variable devices; coloured, fluorescent, phosphorescent and pearlescent inks and optically variable inks; metameric inks, and coloured filters which may be used to view such inks.
When the security device comprises a front to back registration device, such as a Moire pattern, different parts of the device may be applied to the first and second surfaces on both sides of the clear plastics substrate before the opacifying layers of ink are applied.
Alternatively, the security device may comprise a taggant provided within the substrate. The taggant may be invisible to the naked eye, but readable by a reading device through the half-window from the side of the second surface of the substrate.
In one embodiment of the invention, the security document may have a plurality of half-windows which are not covered by opacifying ink. The plurality of half-windows may be provided on both sides of the substrate. For instance, both the first and second surfaces of the substrate may have half-window areas at different transverse locations that are not covered by opacifying ink, said half-window areas being arranged alternately on the first and second surfaces of the substrate. A continuous security device, such as at least one security stripe or thread may extend through the half-window areas to give the appearance of a stripe or thread weaving through the substrate.